Apparatus for assisting in the curing of concrete and for heating

ABSTRACT

Disclosed is a flexible blanket that may be placed over the exposed upper surface of a curing concrete mix. The blanket includes one layer of flexible moisture-impervious material substantially reflective to ultraviolet rays and another layer of flexible material substantially reflective to visible light; also preferably included in the blanket is an electrically-energized flexible heating element that produces heat uniformly over the surface of the bottom layer.

United States Patent Olson 1151 3,676,641 1451 July 11, 1912 ($41APPARATUS FOR ASSISTING IN THE CURING OF CONCRETE AND FOR HEATING [72]Inventor: Wallace A. Ohon, 500 South Kiwanis Avenue, Apt. 315, SiouxFalls, S. Dak.

[22) Filed: Jan. 15, 1971 [2|] Appl. No.: 106,798

Related U.S. Appleatlonllata [62] Division of Ser. No. 770,163, Oct. 24,1968.

[$21 US. Cl. ..219I200, 2 l9l2l3 [5|] InLCI. H05! 1100 [58] Field Search..219/2l3, 34$; 249/78; 25/10 [56] Relerencea Cited UNITED STATESPATENTS 2,566,921 9/195] Briacoe ..2l9/528 X 3,l85,432 5/1965 H t r. Jr.249/711 3,465,121 9/1969 Clark ...2|9/345x 2,961,522 11/1960 Hammer..z19/a45 FOREIGN PATENTS on APPLICATIONS 661,152 1111951 ormamm..219/545 Primary Examiner-C. L. Albritton Attorney-Drake 8:. Crandell[57] ABSTRACT Diacloeed is a flexible blanket that may be placed overthe exposed upper surface of a curing concrete mix. The blanket ineludesone layer of flexible moiamre-impervioua material aubatantiallyreflective to ultraviolet rays and another layer of flexible materialsubstantially reflective to viaible light; alao preferably included inthe blanket is an electrically-energized flexible heating element thatproduce: heat unit'onnly over the surface of the bottom layer.

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500 600 7 00 8 00 900 IO OO H00 200 I00 200 300 4 00 500 InventorWallace A.O|son Attorney APPARATUS FOR ASSISTING IN THE CURING FCONCRETE AND FOR HEATING This application is a division of copendingparent application Ser. No. 770,163, filed Oct. 24, 1968.

The present invention pertains to apparatus for assisting in the curingof concrete. It also relates to electrically-energized heatingapparatus.

Numerous techniques have heretofore been employed in an effort toaccelerate the curing or hardening of concrete. For example, electriccurrent has been actually conducted through the mix itself, in somecases utilizing included reenforcing bars as at least one of theelectrodes. As another approach, hot water or steam has been conductedthrough tubing included in form structures for the purpose of hasteningthe curing process.

Accelerated curing of the concrete is particularly important from aneconomic standpoint in the fabrication of such large structural membersas pre-stressed pilings, beams and other sections used in theconstruction of concrete bridges and the like. These elements typicallyare cast in giant forms that occupy substantial space and in themselvesare very expensive. Absent accelerated curing, the concrete often mustremain in the form for a period of perhaps seven days. Successfulacceleration of the curing can permit removal of the concrete castingwithin 12 to 48 hours, permitting a much greater volume of productionfrom each form. Also in field construction, accelerated curing is moreeconomical in that it pennits form removal after a shorter timeinterval.

In addition to reducing the time required for curing, it is alsodesired, and indeed required, that the member exhibit a certain minimumcompressive strength. That strength typically is specified on the basisof measurements of core samples taken at the end of the initial curingperiod, 3, 7, or 28 days following the original pouring of the concretemix. Some of the prior approaches to the acceleration of concrete havebeen deficient in that the ultimate compressive strength is too low.Other approaches have been excessively costly in terms of the apparatusnecessary to implement them or the cost of the energy employed for thepurpose of heating the mix. Difficulties have also been encountered bysuch efi'ects upon the concrete as creep, shrinkage, camber, warpage,cracks, checks, alligatoring, surface dusting, discoloration, releasefrom the forms and the like.

It is a general object of the present invention to provide new andimproved apparatus for curing concrete that overcome or at leastminimize the aforenoted deficiencies, inefficiencies and difficulties.

Another and particular object of the present invention is to provideapparatus for accelerating the curing of concrete in minimum time whileat the same time maximizing ultimate compressive strength.

A further object of the present invention is to provide new and improvedelectrically-energized heating apparatus.

A feature of the invention is a blanket, which may be laid upon theexposed upper surface of the concrete mix, that includes a first layerof flexible moisture-impervious material substantially reflective toelectromagnetic waves in the ultraviolet and near ultraviolet regions ofthe spectrum and is shaped to overlie the upper surface. Also includedis a second layer of likewise-flexible material that is substantiallyreflective to the electromagnetic waves in the visible regions of thespectrum and is affixed to and across the upper surface of theaforementioned first layer. A related feature is a heating element thatmay be included in the blanket or which may be incorporated intoapparatus inciuding a hollow frame and associated layers respectively ofheat-conductive and heat-insulative materials.

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The organizationand manner of operation of the invention, together with further objectsand advantages thereof, may best be understood by reference to thefollowing description taken in connection with the accompanyingdrawings, in the several figures of which like reference numeralsidentify like elements, and in which:

FIG. I is a perspective view, partially broken away, of a firstembodiment of apparatus for accelerating the curing of concrete;

FIG. 2 is a fragmentary cross-sectional view taken along the line 2-2 inFIG. I;

FIG. 3 is a plan view of an electrically energized heating elementutilized in the apparatus of FIG. 1;

FIG. 4 is a perspective view, partially broken away, of an embodimentalternative to that shown in FIG. 1;

FIG. 5 is a fragmentary cross-sectional view taken along the line 5-5 inFIG. 4;

FIG. 6 is a fragmentary perspective view showing a heating element of akind utilized in the embodiment of FIG. 4;

FIG. 7 is a fragmentary cross-sectional view of still another embodimentof accelerated-curing apparatus;

FIG. 8 is a perspective view, partially broken away, of an electricallyenergized heating apparatus;

FIG. 9 is a fragmentary cross-sectional view taken along the lines 9-9in FIG. 8',

FIG. I0 is a cross-sectional view of a still different embodiment ofelectrically-energized heating apparatus; and

FIG. 11 is a plot of curves illustrating one manner of operating theembodiments depicted by the preceding figures.

FIG. 1 illustrates a giant casting bed or form 20 utilized to mold aT-shaped casting 2] of pre-stressed concrete. The form includes spacedupright panels 22 and 23 that define the vertical member of the T andlaterally projecting sections 24 and 25 that define the underside of thehorizontal portions of the 1. Moreover, sections 24 and 25 are turnedupwardly at their outer ends so as to constitute side walls of the formfor the horizontal T portion. Sections 24 and 25 are joined respectivelyto sections 22 and 23 by still additional slanted sections 26 and 27. Afurther section 23 is joined between the bottom edges of sections 22 and23 to complete the form itself. Section 28 is disposed across aplurality of spaced horizontal beams 29 joined at their respective endsby additional beams 30 from which upwardly project studs 31 on the upperends of which rest sections 24 or 25, as the case may be, on oppositesides of the overall apparatus, As such, casting forms having across-sectional shape as shown, as well as others having a wide varietyof different shapes, are well-known in the pre-stressed concreteindustry. Such forms have at times included fluidcarrying conduitsformed into the structure of sections 22-28 in order to convey heat bymeans of steam or like in an effort to accelerate curing of the concretemix placed into the form.

In FIG. 1, however, heating for the purpose of accelerating curing ofthe concrete is achieved by means of an electrically energized element32 disposed in substantially convective contact with the externalsurfaces of the form. Heating element 32 is itself fabricated so as toproduce heat substantially unifonnly over the form surface. To this end,heating element 32 includes a heat-conductive wire mesh 33 cemented orotherwise affixed directly to the exposed exterior surface of sections22-28. A heat-producing electrically conductive insulatingly coveredlength of resistance wire 34 is folded back and forth across the surfaceof and affixed to mesh 33.

Also disposed at least substantially in convective contact withindividually different ones of sections 22-28, as by being affixeddirectly to mesh 33 in the manner illustrated, are a plurality oftemperature-sensing elements 35 distributed over the outer surface ofthe form in correspondence with separately controlled units ofresistance wire 34. Capillary tubes 36 extend individually from therespective different sensors 35 to thermostatic control units 37. Thelength of each separate unit of resistance wire 34 is selected in viewof the supply source potential and available current capacity, its ownheat dissipation per unit length and the desired heat output per unit ofsurface area. Depending upon the routine selection from among thesedifferent variables, physically separate sections of the folded heatingwire may be joined together in appropriate series or parallelcombinations. However, each such unit is separately energized from aconventional power source, typically a l 10 or 220-volt supply of 60 cpsalternating current, by the actions of the individually differentthermostatic controllers 37 in accordance with their different thermalsettings and the respective different temperatures determined by sensors35. To conserve energy, the side of resistance wire 34 opposite the formpreferable is covered by a layer 38 of heat-insulating material such asasbestos.

As shown in FIG. I for clarity of illustration, wire 34 is folded backand forth with a comparatively wide-spacing between the folds. In actualpractice, the spacing between adjacent folds is preferably more narrowin order to obtain evener heating. In a typical example, the foldspacing is only about 1% inches. The wire, covered with a vinyl or otherinsulating sleeve, is in either solid or stranded form of a conductorsuch as copper typically of about 16 guage. The total length of eachsection is selected to achieve a dissipatation of between and 35, andpreferably about the latter, watts per square foot of form area heated.Mesh 33 in this example is a section of ordinary window screen of about26 guage galvanized steel wire. The mesh is pressed into channels 39 asshown in FIG. 2. At spaced portions along the length of the wire, ametal staple 40 projects through the mesh and clips around insulation 41covering the conductive wire, as depicted in FIGS. 2 and 3. Theresulting construction serves admirably to ensure dissipation of theheat into the concrete mix substantially uniformly over the entireheated area of the form.

Preferably covering the upper surface of mix 21 is a sheet 42 ofmoisture-impervious material in order to prevent evaporation to theatmosphere of water from the mix. Disposed on top of sheet 42 is anotherelectrically energized heating element 43 that in this case again is inthe form of a resistance-wire conductor 44 folded back and forth acrossand affixed to a heat-conductive wire mesh 45. Temperature sensors 46are distributed over mesh in order to sense the temperature in theregion of each different heater unit and supply a corresponding signalto a thermostatic controller 47 that controls the supply of energizingpower to the heating element as indicated schematically in FIG. I.Finally, a layer 48 of heat-insulating material is disposed to overlieconductor 44 and mesh 45. As shown, but a single energizing connectionis provided between controller 47 and conductor 44; in practice,conductor 44 is segmented into a plurality of smaller units eachindividually controlled and energized in response to the respectivedifferent temperature sensors.

Because details in the manner of operating the FIG. 1 apparatus applyalso to others of the structural embodiments to be discussed herein,those other structures will next be described after which attention willbe directed to the operation. Looking then at the apparatus of FIG. 4, aform 50 is shaped to define a pair of adjacent U-shaped channels 51 and52 in which concrete mix is placed to mold a corresponding pair ofsquare beams 53 and 54. The exposed ends of beams 53 and 54 have beenoffset in FIG. 4 to more clearly depict the shape of the channels; inuse, of course, an end plate encloses the ends of each of the channelsand the mix completely fills the space bounded by the forms.

Secured against the exterior heat-conductive channel walls are aplurality of electrically energized heating panels 55 each of which havepower-connecting leads 56. Disposed in thermally conductive contact withthe channel wall adjacent to each panel 55 is a temperature sensingelement 57 from which extends a capillary tube 58 that, in the manner ofFIG. 1, leads to a thermostatic controller which controls the supply ofpower by way of leads 56 to panels 55.

The individual heating panels are composed of a layer 59 of electricallyconductive material such as carbon sandwiched between a pair of layers60 and 61 of electrically insulating material, such as asbestos, with atleast layer 61, which is in convective contact with the wall of thethermally conductive form material, being also substantiallyheat-conductive. Also included between layers 60 and 6l in electricalcontact with spaced portions of layer 59 are electrically conductivestrips 62 to which individual different ones of each pair of leads 56are respectively electrically coupled by connectors 63. The applicationof electric power by way of leads 56 effects the flow of current inconductive layer 59 and the resultant dissipation of heat therefrom byvirtue of resistance heating. To conserve heat, external layer 60preferably is covered with a sheet 64 of thermally insulating material.

Covering the upper surface of the mix from which beams 53 and 54 areformed preferably is a moisture-impervious sheet 65 such as a plastic,metal foil or, conveniently for handling, a thin sheet of aluminum. Aplurality of additional heating panels 66, formed in the same manner aspanel 55, are distributed across sheet 65 with each panel 66 having itsrespective power-connecting leads 67. Similarly in this case, one ormore temperature sensors 68 are in thermal contact with sheet 65 andserve through thermostatic controllers to control or regulate the supplyof energizing power to panels 66.

A modified electrically energized heating panel is shown in FIG. 6wherein a layer 69 of electrically conductive material is sandwichedbetween a pair of layers 70 and 7| that are preferably of plastic inorder to be both highly flexible and mositure-impervious. By reason ofthe latter character of layers 70 or 7 I the resulting heating panelmay, as for example in the case of covering and heating the exposedupper surface of the mix in the forms of either FIGS. 1 or 4, be placeddirectly against the mix. Included between layers 71 and 70, runningalong opposed edges of the panel, are strips 72 and 73 of electricallyconductive material that are in contact with conductive layer 69 so asto serve as spaced electrodes for conducting current through theconductive layer. To that end, connector terminals 74 are secured byrivets 75 to strips 72 and 73 respectively. Instead of an actualsandwich formation, the electrically conductive material in the devicesof either FIGS. 6 and 7 may be directly impregnated in and distributedthroughout a carrier material such as a plastic. Another suitableinsulating material is fiberglass or so-called glass cloth.

To facilitate quick and simple attachment of the heating panels to thewalls of the form, the external surface of layer 70 in this instance iscoated with a contact-type adhesive 76. For ease of handling the panelprior to installation, adhesive layer 76 is covered with a thin sheet 77of a protective material such as paper. To apply the panel to a surfaceof the form, the installer merely peels off layer 77 and presses thepanel directly against the wall of the form whereupon adhesive 76 holdsit in place. In this way, defective panels may be quickly substitutedand, after use with temporary forms such as those often set up in fieldconstruction, the panels may be removed for subsequent use elsewhere.The flexibility of layers 69-7] together with the use of adhesive 76permits attachment of the panels to form surfaces of irregular shapes.

For particular use directly upon exposed concrete mix surfaces, such asover the top of the mix placed into the forms of FIGS. 1 and 4 or on theupper surface of concrete slabs in the case of pavement or sidewalkconstruction, the heating unit of FIG. 7 is in the form of a lightweightflexible blanket 78. Like in the case of FIG. 6, the underside ofblanket 78 includes an electrically conductive layer 79 of a materialsuch as graphite across spaced portions of which electrically conductivepower-connecting strips (not shown) are connected. Conductive layer 79is sandwiched between a pair of layers 80 and 81 with at least layer 81being both heat-conductive and moisture-impervious. For simplicity offabrication, both layers 80 and 81 are in this case formed of plastic.Secured to and disposed over the surface of upper layer 80 is a sheet 82of heat-insulative material preferably composed of foam vinyl or otherresilient substance. Imparting strength to blanket 78 against tearing ofsheet 82 is a preferably included web 83 or reenforcing material formedby interlaced strings of a tough material such as nylon. For purposes tobe further discussed hereinafter, at least the upper, exposed surface 84of sheet 82 is a bright color, such as white or yellow, in order to behighly reflective to light or electromagnetic waves in the visibleportion of the spectrum. At the same time, preferably both layers 80 and81, as well as conductive layer 79 when composed of graphite, are of ablack color in order to be highly reflective of electromagneticradiation in the ultraviolet and near-ultraviolet regions of thespectrum.

To enable placement over a generally flat and horizontal surface to beheated, the apparatus of FIG. 8 includes a hollow, rectangular frame 85composed of channels 86 secured together and preferably formed of alightweight material such as aluminum. Diagonally opposite corners offrame 85 and oppositely spaced other portions of the frame areinterconnected by struts 87 in order to give rigidity to the unit. Theresulting frame thus has its major dimensions horizontal and itssidewalls define both an open top and an open bottom.

Disposed between those sidewalls across the open top is a layer 88 ofheat-conductive material. In this case, layer 88 is a wire mesh of thekind shown and described with respect to FIG. 3 secured around itsperiphery to the upper surface of channels 86. Similarly, anelectrically conductive resistance wire 89 having an electricallyinsulative covering is distributed across the surface of mesh 88 bybeing folded back and forth and pressed into channels formed in themesh. Located so as to overlie mesh 88 and conductor 89 is a sheet 90 ofheat-insulating material, in this case a sheet of conventional buildinginsulation having an insulating filler 91 and an aluminum-foil backing92 which is placed directly over the heating wire 89. Disposed acrossthe upper or outer surface of filler 91 is a thin sheet 93 preferablyagain of a lightweight material such as aluminum. Sheet 93 thus also ismoisture-impervious so as to shield against the effects of precipitationin an outdoor environment.

In use, the FIG. 8 unit may be placed directly over a horizontallydisposed quantity of concrete mix such as exists in the case of theformation of a slab. In this case, the actual heating unit, composed ofmesh 88 and resistance wire 89, is spaced by the width of channels 86from the mix. This is particularly advantageous in certain fieldapplications wherein the concrete mix includes an extremely courseaggregate such as stones of considerable size. In that situation, astone near the upper surface of the mix can act to concentrate andreflect back into the heating element an intense quantity of heat, insome cases sufficient to scorch or even destroy a section of the heatingelement when the latter is placed directly upon the mix surface. Thatresult is avoided with the FIG. 8 apparatus by virtue of the fact thatthe heating element is spaced a short distance away from the upper mixsurface and thus is not in direct thermal contact therewith.

An auxillary use for the FIG. 8 apparatus is particularly valuableduring conditions of freezing weather. With the ground frozen, it is, ofcourse, difi'lcult to dig into the earth in order, for example, toexcavate the earth in preparation for pouring a concrete slab. However,simply by placing the FIG. 8 apparatus over the region to be excavatedand energizing resistance wire 89, sufficient heat is dissipated intothe ground comparatively quickly to thaw the earth even to a depth ofseveral feet. In a typical practical embodiment for that purpose, thesizing and layout of resistance wire 89 is chosen so as to dissipateapproximately 70 to 100 watts per square foot. As compared with laying ablanket such as that shown in FIG. 7 directly upon the ground, theapparatus of FIG. 8 also is advantageous when used from ground thawing,again because channels 86 space the heating element a short distanceabove the ground so that large rocks or other objects on or near theearth's surface cannot concentrate and reflect back sufficient heat todamage the heating unit. In use of this apparatus for thawing theground, it is also advantageous in that the thawing occurs with adispersal of the moisture that is unfrozen. This contrasts with the useof steam for ground thawing which results in the earth becoming muddy.

FIG. illustrates a heating unit generally similar in shape to that ofFIG. 8 and particularly adapted for the curing of concrete slabs moldedin a horizontally disposed form 94 having its major dimensionshorizontal and its sidewalls 95 defining an open top. In at least mostcases involving the formation of a slab whose thickness is smallcompared with its length and width, sufficiently uniform acceleratedcuring may be effected by heating only the exposed upper surface of themix, as in the case of mix 96 in FIG. I0. To this end, the heating unitincludes a rectangular frame 97 in this case formed of angle iron shapedto mount upon the upper periphery of sidewalls 95. Stretchedhorizontally across the frame so as to overlie the upper surface of mix96, across the open top of form 94, is a layer 98 of a heat-conductivematerial such as a sheet of aluminum. An electrically energized heatingelement is disposed substantially in convective contact with layer 98,and in order to produce heat substantially uniformally over this surfacethe element is again composed of a length or lengths of resistance wire99 folded back and forth so as to be distributed over layer 98.Overlying heating element 99 is a layer I00 of heat-insulative material.To strengthen the entire assembly as well as to permit rougher handling,layer I00 preferably is covered by a sheet 10! of a material such asaluminum the outer edge portions of which are bent downwardly andsecured to frame 97.

Turning now to the mode and manner of operation of the various differentapparatus described above, it is helpful first to consider certainbackground material. The binding material utilized in the formation ofconcrete is commonly called cement. The concrete itself is the resultinghard mass that is formed from a mixture of cement, certain additives, anaggregate and water. The aggregate constitutes a filler and typicallyconsists of various different proportions of sand, gravel and stone. Theadditives may include air entraining compounds and materials that areintended to produce better or quicker initial setting of the mix. Atypical mix is composed of 94 pounds cement, I69 pounds of moist sand,336 pounds of moist gravel and 4% gallons of water. In general, mostbuilding construction involves the use of five to seven pound sacks ofcement for each cubic yard of aggregate.

The cement itself is manufactured by driving out the moisture fromnatural chemical compounds in a high temperature kiln. Such compoundsare composed primarily of lime, silica, certain clays and alumina. Theresulting mass derived from the kiln is a solid clinker thatsubsequently is ground and screened to provide a powdery material. Indriving out the moisture from the natural chemical by the use of heat,the chemical and physical change in the natural material involves theabsorportion of energy by the material. The molecular structure issignificantly changed so as to no longer have a link of water; it isdehydrated. Thus, the resulting cement, even though in powder form,retains latent energy. When this energy subsequently is released, aswater is added to the mix, the reaction is exothermic so that the mixgives up heat. That is, water is taken back into the cement mixture as ahydration reaction so as again to become an integral molecular part ofthe cement as a result of which a binding material is created. Theenergy given off during the hydrating chemical reaction is sometimestermed the heat of hydration.

To obtain proper compressive strength in freshly mixed and placedconcrete, it is necessary that moisture be retained in the concrete in asufficient amount during its curing to allow complete chemical reaction(hydration) between the water and the cement to take place. Thus, one ofthe purposes of placing moisture-impermeable sheet 42 in FIG. I over theupper surface of the mix is to protect the as-yet-unformed upper surfacefrom loss of moisture. When, instead, moisture is lost from a mix byreason of improper bedding under the mix, capillary attraction to theforms or covers or evaporation to the atmosphere as in the case whereheated air is passed over the surface in attempted accelerated curing,the desired chemical balance in the ultimate concrete is upset. As aresult, cracks and other undesirable surface blemishes typically appear.

In the usual concrete mix, more than a sufficient amount of moisture isincluded initially to properly complete the hydration process. However,when the care indicated is not taken to retain the moisture in theconcrete so that the hydrating action is not permitted to becomecomplete, the internal structure of the resulting concrete is disruptedas a result of which it exhibits insufficient compressive strength.Another concern in the placing of concrete arises during cold weather.When the temperature of fresh concrete is low, as may occur in coldweather, the initial setting is delayed as a result of which the mix mayfreeze so that proper hydration cannot take place. Even when initiallyset but not yet well cured, exposure of the mix to a freezing atmospherewhile the compressive strength is still low may result in rupture of theinternal structure. Consequently, the different heating apparatusdescribed not only is advantageous from the standpoint of acceleratingcuring but also in permitting the pouring and formation of concretestructures during cold weather. This is especially significant in thecase of huge forms such as that illustrated in FIG. I; by permittingyear-round operation even though located outdoors, the return from theinvestment in the form is, of course, substantially increased.

A different effect, most likely to occur in the summer, can also causedisruption of the internal structure of the concrete. The naturalhydration action can be undesirably disturbed by the addition ofexcessive heat, particularly when the excess persists for a period ofseveral hours. This disturbance may be encountered as a result of directrays emanated by the sun. The damaging rays include both those invisible portion of the spectrum and those in the ultraviolet and nearultraviolet regions. The consequence of permitting sun rays to strikethe concrete in an excessive amount is to effect a regression ofcompressive strength gain during the curing period with a consequentloss in ultimate compressive strength. This, of course, can betroublesome with regard to forms such as those in FIGS. 1 and 4 that,because of their large size, often are located in the open. The troubleis also encountered necessarily in the pouring-in-place of concreteslabs during the construction of highways and sidewalks.

It is, then, to the end of avoiding such difficulty as a result of thedirect rays of the sun that, in FIG. 7, sheet 82 of blanket 78 iscolored so as to reflect the visible light waves and layers 80 and 81are formed of a black-colored plastic so as to inhibit penetration ofthe ultraviolet and near ultraviolet rays. The use of blanket 78 mayserve several purposes at the same time, particularly in some climatessuch as those at high altitudes where the daytime hours may be featuredby comparatively high temperatures with bright sunlight while during theimmediately following night the temperature drops to a value belowfreezing. That is, blanket 78 forms a moisture-impervious seal over theconcrete surface in order to prevent escape of the necessary water ofhydration, its heating element when energized functions to preventfreezing of the curing mix and to accelerate curing and, at the sametime, the reflective features of the blanket prevent overheating by sunradiation.

Generally speaking, the hydration process which is an essential part ofhardening or curing of the concrete continues at a significant rate fora number of days. As related to unaccelerated curing at normal ambienttemperatures, the compressive strengths of the concrete typically aremeasured and rated in terms of the strength at fixed intervals of timesuch as at 3 days, 7 days and 28 days following the time of pouring. Areasonably-high ultimate compressive strength would be a value over6,000 pounds per square inch.

As indicated, the overall aim of accelerating the curing of concrete isto cause the concrete to reach at least near its ultimate compressivestrength in a shorter period of time, and it is to that end that anexternal heat source is employed to apply heat to the mix. The functionof the heat is to cause the water to be hydrated into the cement morequickly as a result of which what conventionally would be termed 3, 7 or28 days compressive strengths are obtained in a fraction of that time.it is believed that greater hydration of water results duringaccelerated curing. The consequent formation of a greater bulk of cementpaste, in turn, forms a less thick coat around each aggregate particle,and this accounts for the greater compressive strengths exhibited in theend.

However, an attempt to obtain even greater acceleration of the curing,by dissipating very large quantities of heat into the mix in order togreatly elevate its temperature, leads to an actual regression incompressive strength. In order to avoid the occurence of such regressionwhile at the same time obtaining maximum-possible compressive strengthin the minimum time interval, the manner in which the heat is applied iscarefully controlled. This is the primary function of the temperaturesensors and thermostatic controllers described above. Optimum resultsare obtained by first applying the heat substantially uniformly over theouter surface of the mix and then terminating application of the heatwhen the measured temperature reaches a preselected level. That level issuch that the immediately subsequent exothermic heating, which continueswithin the mix, further increased the mix temperature only to apredetermined maximum value. That maximum value corresponds to theattainment of maximum ultimate compressive strength at the end of thecuring period. Thus, the application of the heat is tenninated earlierthan might otherwise be the case when a temperature level is reachedthat anticipates the subsequent further exothermic heat rise so that theultimate temperature the mix reaches is the most consistent withobtaining the highest compressive strength in the end.

While the ultimate compressive strength of concrete varies with respectto cements obtained in different geographical sections of the country,and even to some extent in the output from a single cement plant, themaximum value to which the mix temperature is caused and permitted torise, including the increase due to exothermic reaction continuing afterterminating the heating step, is found to be generally between 150 and160. In order to anticipate the subsequent exothermic temperature rise,it is also found that the application of heat from the external sourcemust be terminated when the temperature rises to a level of about I40.In addition, it is found preferable to include what may be termed apre-setting period before heat from the electrically energized heatingelement is applied. That is, the application of the heat is delayeduntil the initially fluid mix reaches the semi-fluid condition typifiedas that when the exposed surface may first be meaningfully smoothed bytrowelling. Additionally, the form may be preheated where necessary sothat its temperature initially is approximately the same as that of themix.

A typical time-temperature plot of such a heat cycle appears in FIG. 11from the accelerated curing of concrete formed in a giant-T casting bedsimilar to the apparatus of FIG. I. The same bed also includedoriginally installed duct work that had been utilized for the purpose ofconveying steam in an effort to accelerate the curing. The four curvesgrouped rather closely together represent temperatures as measured inindividually different sections along the length of the form. The othercurve, that which appears to depart from the norm and is the lowest atthe 4 o'clock position, indicates a measurement taken on a vertical testcylinder. lts lesser mass permitted a small loss of heat of hydration asa result of which its temperature was slightly reduced afterdeenergization of the heating element.

A type II cement was utilized in the mix that was placed into thecasting bed at a mix temperature of 70 F. The ambient temperature of theatmosphere surrounding the bed at the time of placement was 50 F. Forthe first two hours after placement, no external heat was applied fromany of the heat ing elements. After that 2-hour period, at 5:00 p.m.,the heating elements were fully energized so as to dissipate heat at arate of approximately 35 watts per square foot. After approximately 4%hours, at 9:30 p.m., the mix temperature reached an average value ofabout F. as a result of which the then mostatic controls automaticallydeenergized the heating elements. However, the subsequent exothermicheating as a result of the continuing hydration process caused thetemperature to continue to rise to a level of approximately at which thetemperature remained on through the night. It appears that the describedelectric curing results in a phenomena within the mix that produces acontinuation of exothermic reaction throughout a complete initial curingcycle that continues long after deenergization of the heating elements.

A core sample taken at the end of the initial curing period shown, 5:00am. the next morning, exhibited a compressive strength of 4200 poundsper square inch. This compared favorably with a core sample thatpreviously had been taken after the same time interval from a mix thecuring of which was accelerated by utilizing the steam conduits affixedto the mold. That sample had yielded a eolnpresslve strength of only3,500 pounds per square inch. Repeated tests of core samples, taken atthe end of 28 days following the initial accelerated curing according tothe schedule shown in FIG. 11, resulted in the attainment of compressivestrengths consistently about 800 pounds per square inch above that whichpreviously were regularly obtained by the use of steam curing in thesame forming apparatus.

In other comparative tests, application of the heat in the manner of andaccording to the schedule herein described has resulted in concretethat, after only 72 hours, exhibited a compressive strength 600 poundsper square inch greater than the strength exhibited by concrete curedfor 7 days in accordance with so-called standard laboratory curing. Thatis a form that previously might have been able to turn out but onecasting per week may, by use of the principles herein discussed, haveits production increased to two or three castings per week. Suchincreased productivity results in lower curing cost, and the closecontrol of the curing as a result of the measured application of theheat results in more uniform high compressive strengths in successivelyproduced castings.

The illustrated apparatus may take many different shapes and forms incorrespondence with the production of a wide variety ofdifferently-shaped concrete structures. While steel forms have beenillustrated in FIGS. 1 and 4 as are typical to hold the heavy weights ofconcrete mix involved, other materials may be more advantageous for usein field construction. Wood, appropriately treated to reduce capillaryattraction to the moisture in the mix, is a typical form material whichis heat conductive. Another advantageous material is fiber glass inwhich case the heating-element resistance wire conveniently may beimbedded directly therein. In addition to forming structural members ashave herein been illustrated, the techniques are equally applicable tothe formation of complete structures such as concrete cattle-feedingbunkers.

Although the use of integral sheets of insulation have been particularlyillustrated for the purpose of minimizing heat loss from the side of theheating element opposite the concrete mix, such insulation may be formedin any convenient manner. For example, after affixing the heatingpanels, such as those in FIG. 6, directly against a form wall,insulation then may be simply sprayed into place over the heatingpanels. This is particularly advantageous, for example, when affixingthe panels to various steel form sections in the fabrication of verticalbuilding walls and wherein the panels are of a variety of sizes andcontours.

Numerous features of the present invention are claimed in theaforementioned parent application. Other such features are claimed incopending applications Ser. No. 106,784, filed Jan. 15, 197i and Ser.No. ll0,5l2, filed Jan. 28, 1971, which also are divisions of the parentapplication.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects and, therefore, the aim in the appended claims is tocover all such changes and modifications as fall within the true spiritand scope of the invention.

lclaim:

l. A flexible rollable heat control blanket comprising:

a first layer composed of a thin, flexible, moisture-impervious,heat-conductive and electrically-insulative plastic material; secondlayer contiguous with said first layer and composed of a thin, flexibleand electrically conductive material productive of heat in response tothe application of an electric potential across opposing edge portionsthereof;

a third layer contiguous with said second layer and composed of a thin,flexible moisture-impervious and electricall? insulative plasticmaterial, at least one of said first an third layers being of the colorblack so as to Il'll'llbli penetration of electromagnetic waves in theultraviolet and near-ultraviolet region of the spectrum;

a fourth layer contiguous with said third layer and composed of acomparatively substantially thicker, flexible heat-insulative andresilient material of foam vinyl substance, the exposed surface of saidfourth layer opposite said third layer being of a very light colorincluding one of the colors yellow and white so as to be substantiallyreflective to electromagnetic waves in the visible region of thespectrum;

and a flexible reinforcing web, composed of interlaced strings of nylonexhibiting substantial tensile strength, buried within said foam vinylsubstance and spaced from and generally parallel to said exposed surfaceand said third layer.

1. A flexible rollable heat control blanket comprising: a first layerComposed of a thin, flexible, moisture-impervious, heat-conductive andelectrically-insulative plastic material; a second layer contiguous withsaid first layer and composed of a thin, flexible and electricallyconductive material productive of heat in response to the application ofan electric potential across opposing edge portions thereof; a thirdlayer contiguous with said second layer and composed of a thin, flexiblemoisture-impervious and electrically insulative plastic material, atleast one of said first and third layers being of the color black so asto inhibit penetration of electromagnetic waves in the ultraviolet andnear-ultraviolet region of the spectrum; a fourth layer contiguous withsaid third layer and composed of a comparatively substantially thicker,flexible heat-insulative and resilient material of foam vinyl substance,the exposed surface of said fourth layer opposite said third layer beingof a very light color including one of the colors yellow and white so asto be substantially reflective to electromagnetic waves in the visibleregion of the spectrum; and a flexible reinforcing web, composed ofinterlaced strings of nylon exhibiting substantial tensile strength,buried within said foam vinyl substance and spaced from and generallyparallel to said exposed surface and said third layer.